Power line communication (“PLC”) systems use electrical power lines to exchange information between electronic devices in a network, such as between computers or shared electronic devices. Power line modems (“PLMs”) are used in PLC systems to connect electronic devices in a home, office, building, or inter-building computer network and similar applications.
PLMs typically have a receiver and one or more transmitter line drivers that are coupled to the power lines through a coupling network. The PLM is often powered by a power supply(s) coupled to the power lines. The power lines typically operate at a voltage level that is dangerous, such as 120 VAC or 230 VAC. It is important to protect the user of devices connected to the power supply from electrical shock or injury.
FIG. 1 is a diagram of a prior art PLC system 100 using parallel signal injection. The power lines, also known as “mains lines” or simply “mains,” 102, 104 are arbitrarily designated “line” (“L”) 102, and “neutral” (“N”) 104 for purposes of discussion. The power lines 102, 104 typically operate at a relatively high AC voltage to provide electrical power to devices and appliances. AC power from the power lines 102, 104 is provided to a power supply 106 along power supply leads 108, 110. An EMI filter capacitor 112 is provided across the power supply leads 108, 110 to suppress electromagnetic interference (“EMI”) that might otherwise enter the power supply 106 and hence the PLM 114. The EMI filter capacitor is typically greater than about 1 nano-Farad (“nF”). A coupling network 116 couples the signals received by, and transmitted from, the PLM 114 to the power lines 102, 104. The PLM 114 is designed to operate in a half-duplex system, where the PLM can either transmit or receive data, but does not transmit and receive data at the same time.
FIG. 2 is a simplified circuit diagram of a conventional coupling network 118 using transformer-coupled parallel signal injection. A transmitter line driver 120 and a receiver 122 are coupled to the power line interfaces 102′, 104′ through a transformer 124, capacitors 126, 128, 130, and an inductor 132. The capacitor 126 between the output 134 of the transmitter line drive 120 and the transformer 124 and the capacitor 128 between the input 136 of the receiver and the transformer 124 are chosen to have a value that passes signals at a selected frequency (the “signal frequency”). An exemplary signal frequency is 100 kHz, alternative signal frequencies are generally between the line frequency (e.g. about 50 or 60 Hz) and about 100 MHz.
Generally, the values of the capacitors 126, 128 are selected to provide relatively low impedance at the signal frequency. The values for the capacitor 130 and inductor 132 in series are selected to provide a resonant short circuit at the signal frequency (i.e. where the total impedance of the series combination goes to about zero at the signal frequency). This circuit is commonly known as a “series L-C circuit.”
The transformer 124 isolates the high voltage of the power lines from the transmitter line driver 120 and receiver 122. Additionally, a first voltage-limiting device 134, such as a metal-oxide varistor (“MOV”), limits the voltage across the power line interface ports 102′, 104′. For example, in a system designed to operate at a power line voltage of 230 VAC, the MOV 134 is selected to limit the voltage across it to less than 275 V. In other words, the MOV 134 transitions from a high resistive state (essentially an open circuit), to a low resistive state (typically a few ohms) at 275 V. The MOV 134 limits the voltage that can appear across the input winding of the transformer.
A second voltage-limiting device 136, such as an avalanche diode or a Zener diode, limits the voltage at the PLM side of the transformer 124. The second voltage-limiting device 136 is commonly known as a “transient voltage suppressor” (“TVS”), and conducts (i.e. becomes essentially a short) at a second selected voltage, typically about 5 V for conventional receivers and transmitters.
FIG. 3 is a simplified circuit diagram of another conventional coupling network 140 using transformer-coupled parallel signal injection and two transmitters. The receiver 122′ receives a differential input from both sides of the secondary winding 142 of the transformer 124. The TVS 136 is connected across the inputs 144, 146 of the receiver 122′. The transmitter line drivers 120, 120′ have input signals 148, 150 one hundred and eighty degrees out of phase that drive the secondary winding 142 of the transmitter, and essentially double the power of the transmitted signal, compared to a PLM using a single transmitter line driver of similar design.
FIG. 4 is a simplified circuit diagram of a conventional coupling network 160 using capacitively coupled parallel signal injection. A shunt inductor 162 shunts the power line voltage, which is typically at 60 Hz or 50 Hz (i.e. at a much lower frequency than the signal frequency) that is not blocked by the capacitor 130′ and inductor 132′, which form a series L-C circuit. The TVS 136 is connected across the shunt inductor 162 to protect the transmitter line driver 120 and receiver 122 from transient voltage spikes that might otherwise be coupled through the coupling capacitors 126, 128.
However, using multiple PLMs with parallel signal injection, whether transformer coupled or capacitively coupled, undesirably loads the signals to and from a PLM when used with a standard power supply incorporating a filter capacitor.